In a cellular telephone system, service is provided to cellular telephones by providing many adjacent geographic areas, called cells, each of which is served by a control station called a base station. Each base station provides a broadcast control channel to provide service to all idle, authorized cellular terminals within the boundaries of the cell, as well as sufficient transmit/receive carrier pairs to support some number of active mobile terminals. One of the primary functions of the mobile terminal in idle mode is to monitor the control carrier within the cell in which it resides. This carrier contains cell specific information (broadcast information) required by the mobile terminal to operate within the cell, as well as messages directed to specific terminals to inform them of incoming calls. These messages are commonly known as pages, and usually result in the ringing of the terminal and eventual establishment of an active connection between the terminal and the calling party.
In Time Division Multiple Access (TDMA) cellular systems, such as the well-known IS-136 (D-AMPS) and ETSI GSM systems, the control channel is one of multiple independent channels on a single carrier frequency. In TDMA systems, carriers are divided in time into fixed intervals known as time slots, as shown in FIG. 1. Each time slot carries a burst of data encoded in the R.F. signal.
These time slots are numbered sequentially in time, and those slots repeating at some fixed integral number of slots (m) comprise what is known as a channel. The repeating interval containing m slots is known as a frame, and slots within the frame are numbered 0,1, . . . m-1, as shown in FIG. 2.
In conventional TDMA systems, the mobile terminal receives and transmits only during the period of one time slot (channel) per frame. Frames and time slots are numbered in the same way for both receive and transmit, but the time reference is shifted between them so that the terminal does not have to receive and transmit at the same time. This eliminates the need for a costly and complex duplexer, a device that allows the same antenna to be used for transmit and receive at the same time.
The control channel in a TDMA system is carried on one time slot on a single carrier. On the control channel, there are two types of data. The first type of data is cell-specific information, which includes synchronization and timing information, and cell-specific broadcast control information. The second type of data is mobile-targeted data such as pages and messages used in call setup when establishing an active connection with a mobile terminal. A typical control channel structure consists of a repeating series of frames, known as a multi-frame. The control frame structure used in GSM, for example, is shown in FIG. 3.
In FIG. 3, the bursts F are frequency correction bursts (1 time slot each). Bursts S are synchronization bursts (also 1 time slot each). Bursts BCCH carry broadcast control channel data (messages encoded over 4 time slots), and bursts CCCH carry common control channel data (also messages encoded over 4 time slots). Bursts I are idle bursts of 1 time slot each. This structure exists in time slot 0 of the control carrier in a GSM cell. The components are utilized as follows.
The Frequency Correction Burst is a burst of fixed frequency (no data content) modulating the carrier frequency, and occurs 5 times in the 51 frame structure. Its purpose is to aid the mobile terminal in locating the control carrier in a cell when it does not know the frequency of the carrier, and to provide initial frequency synchronization between the mobile and the cell. The mobile terminal will typically scan possible carrier frequencies for this fixed frequency burst (after demodulation). Once found, the terminal knows it has found a GSM carrier frequency and can make a coarse frequency adjustment based on this signal. It also knows the rough (coarse) timing of time slot 0 in the frame, as this burst always occurs on time slot 0.
The Synchronization Channel burst contains information on the timing structure of the cell (frame numbering) and additional synchronization information to allow fine frequency and timing adjustment. It identifies its own frame number within the 51 frame multi-frame structure, as well as which occurrence of the multi-frame structure is currently being transmitted. The terminal receives this burst in the frame following the receipt of the frequency correction burst.
The Broadcast Control Channel contains information about the cell that the terminal must have to communicate with the cellular system from within the cell. The channel carries blocks of data (messages) coded over four sequential frames in four bursts. Up to 8 such messages are required to carry the entire data content of the broadcast control channel. The messages are thus numbered from 1 to 8, and identified by taking the occurrence number of the multi-frame, derived from the synchronization channel, modulo 8.
The Common Control Channels contain messages coded over four frames, as with the broadcast control channel. These channels carry the messages targeted to specific mobile terminals, such as pages. There are several channels allocated, and each mobile terminal receives only one channel. The mobile determines which channel to receive based on data obtained from the broadcast control channel and its own internal identification number, known to itself and the cellular system, by which it is paged.
When there is sufficient capacity to support all mobile terminals in a cell with a single control channel structure, the structure above is used on time slot 0. If additional paging capacity is required, the above channel structure can be repeated on other channels (time slots) on the control carrier, but with the frequency correction burst and the synchronization burst excluded. In this case, the mobile terminal will initially receive the channel on time slot 0 to synchronize to the system, but may then monitor the control information on one of the other time slots for broadcast and other information. Synchronization is maintained on these channels by the process of decoding the broadcast information and common control channels. The terminal is not required to monitor more than one time slot at a time. Further details of conventional control channels are described in the ETSI GSM specification, part 5, sections 01 and 02, and in D-AMPS specification IS-136, which are incorporated by reference.
In communications systems where very low signal levels are expected at the mobile terminal, new requirements are placed on the control channel. Such a concern arises in satellite based systems, where distance and restrictions on available peak and average transmitter power must be accommodated. At low signal levels, the amount of signal blockage (by the head, body, trees or other obstructions) that can be accommodated before the signal level at the receiver is insufficient to allow correct reception of the signal is very small. While the user can assist in assuring a good signal path when in active mode (while establishing or maintaining a call), user assistance cannot be expected when the terminal is idle. Such a terminal is said to be disadvantaged when it is attempting to receive a signal below the threshold required for successful reception at normal signal levels. In such systems, a means for providing a boosted signal for synchronization and paging functions has been devised.
In this method, some bursts on the control channel are transmitted at higher power and with additional coding to achieve greater likelihood of being received successfully by the terminal when it is disadvantaged. A control channel of this type, as described in the ACeS air interface specification, is shown in FIG. 4.
In FIG. 4, H designates a high power burst, S designates a synchronization burst (1 time slot), BCCH designates broadcast control channel data (messages encoded over 4 time slots), CCCH designates common control channel data (messages encoded over 4 time slots), and I designates an idle burst.
In this channel structure the multi-frame is 102 frames long. All bursts on the channel are transmitted at the same power level, except for the four high power bursts, which are transmitted at a level of about 7 db over the other bursts. This characteristic allows the mobile terminal to locate the carrier containing the control channel by measuring the energy over time on a possible carrier frequency, and looking for the unique pattern or "power profile" presented by the four high power bursts. The asymmetrical nature allow the mobile to obtain both coarse timing for the location of the time slots of the channel, as well as determine which burst is associated with frame 0 of the channel. This replaces the function of the frequency correction burst in GSM.
The first high power burst in this channel also has the unique characteristic of providing a known data sequence, which the terminal can decode and use to obtain fine frequency and timing adjustment. The other three high power bursts are used to carry highly coded data. Coding provides an additional 12 dB of gain over the other bursts on the channel, giving these bursts an effective signal strength (as perceived by the terminal) of approximately 20 db above the remainder of the channel. This coding, however, reduces the effective data capacity per burst from 46 bits/burst for the normal power bursts to 7 bits/burst for the high power bursts.
In a system utilizing this control channel structure, there may be many such control carriers transmitted using the same power source. An example would be a satellite providing coverage to many cells on the earth (also known as beams), each with its own control carrier. In order to limit the peak power requirement for the power source, it is desirable to have only one high power burst being transmitted at any one time across all the beams being fed by the same power source. With this structure, by staggering the start time of the multi-frame between beams, there are 25 staggers in which no two beams are transmitting a frame containing a high power burst at the same time. With a frame structure containing 8 time slots, as in GSM, and by further staggering the timing between beams on slot intervals, there are 8 times 25, or 200 possible staggers between beams that result in no two beams transmitting a high power burst at the same time.
In this system, the disadvantaged mobile terminal receives page messages coded on the high power bursts. To receive broadcast information, it must become advantaged (move to a good receiving position) before it can receive the broadcast channel. It must also become advantaged to establish and maintain an active connection.
In the above systems, specifically when addressing the mobile terminal in a low signal system such as a satellite based system, there are two characteristics that are undesirable. First, the data carrying capacity of the high power bursts is very low, limiting the number of mobile terminals that may be paged per second in a beam when this is the primary paging method. Second, the mobile terminal must become advantaged in order to read the broadcast information on the channel. Although this data changes infrequently, current information is necessary in order to establish an active connection. This means that when a terminal receives a page and the user moves to an advantaged position, the terminal must first verify the accuracy of the broadcast data it last read by rereading the broadcast data from the channel before attempting to establish the connection. This results in a delay in the set up of the call.
As described above, a broadcast control channel is transmitted by each control station (e.g., base station or satellite beam), and is monitored by all idle mobiles in the service area associated with a particular control station. The ACeS satellite phone system, which is based on the GSM air interface standard, includes a broadcast control channel which preferably has synchronization and data-carrying capacity requirements in addition to the requirements of the GSM broadcast control channel. In the ACeS system, the BCCH preferably has the following additional qualities beyond the BCCH in the GSM system:
1. The BCCH can be transmitted at a "boosted" level (at an increased power level) to reach disadvantaged terminals (i.e., mobiles that cannot monitor the normal-power BCCH due to signal blockage).
2. The boosted signal allows the terminal to synchronize to the channel, provide common broadcast information to the terminal, and provide a terminal-specific call alerting function.
3. The terminal can preferably monitor both the boosted signal and the normal signals when not disadvantaged.
4. The transmission of boosted signals in the system is preferably distributed such that a maximum transmission power is not exceeded.
In ACeS, one channel on the control carrier provides synchronization and low data rate information to disadvantaged terminals. Since the data rate on this channel is too slow to meet the data rate requirements for boosted power alerting, it is desirable to provide and additional channel for this purpose.
In the GSM system, the terminal receives only one channel at a time, thereby preventing the terminal from receiving sufficient data to meet the above requirements.